Juno RPC handler code tidying

[stranger80]

• Refactor the RPC handler code in Juno
  • improve breakdown into code files, etc.
  • refer to @kirugan for more suggestions re refactoring
• Increase coverage of automated tests
• Implement a pattern of generating RPC handler code scaffolding from OpenAPI spec

[Aneeque]

Implement a pattern of generating RPC handler code scaffolding from OpenAPI spec

Note the starknet spec is written in OpenRPC not OpenAPI and I don't think there is a good code generator in go for OpenRPC